1. Field of the Invention
This invention relates to the tempering of glass sheets and, more specifically, to the cooling of hot glass sheets involved in their tempering while conveying the sheets through a cooling area immediately downstream of a furnace wherein the conveyed sheets are supported on a gaseous bed with their major surfaces out of contact with solid members. This cooling involves a system for supplying gas in heat exchange relationship and/or supporting relationship to a sheet or ribbon of glass. The support system is particularly adapted for handling glass in sheet or ribbon form without marring or otherwise producing uncontrollable deformation in the major surfaces of the sheet, even when the glass is at a deformation temperature.
Tempering produced by heating a glass sheet above its annealing range and then rapidly chilling its surfaces to below the strain point while the interior is still hot and continuing the rapid chilling until the entire glass sheet cools to below its strain point causes the glass sheet to develop a skin of compression stress that surrounds the glass interior which is stressed in tension. Such a stress distribution makes the glass sheet much stronger than untempered glass so that tempered glass is less likely to fracture than untempered glass when struck by an object. Furthermore, in the less frequent times when an outside force is sufficiently large to cause tempered glass to fracture, tempered glass breaks up into a large number of relatively smoothly surfaced, relatively small particles which are far less dangerous than the relatively large pieces with relatively jagged edges that result from the fracture of untempered glass.
In fabricating glass through known manufacturing techniques of bending, tempering, annealing or coating and combinations of such techniques to form end products having characteristics and uses different from the original product, it is necessary to heat the glass sheets to a temperature above that at which the major surfaces or the contour thereof is changed by deforming stress on contact with solid members. Where it is desired to strengthen the glass, it is further ncessary to cool the glass sheets rapidly from such a deformation temperature to a lower temperature below the annealing range of the glass. The effectiveness of such strengthening is improved by an increase in the rate at which heat is removed from the surfaces with respect to the center of the thickness of the glass sheets.
Efficient glass sheet fabrication involving the techniques previously mentioned requires that the glass sheets undergoing treatment be conveyed while hot. The need to convey glass sheets at high temperature has involved undesirable deformation or marring of the major surfaces of glass sheets undergoing treatment due to physical contact of its major surfaces with supporting and conveying apparatus while the glass is at elevated temperatures. Glass sheets have been supported on gaseous beds to overcome the defects of deformation and marring due to physical contact of their major surfaces with solid members at elevated temperatures. Glass sheets have been conveyed through these gaseous beds by supporting the sheets at a small oblique angle to the horizontal and engaging the lower edges thereof with the peripheries of rotating driving discs.
Attempts to cool the glass surfaces rapidly has involved the development of modules for supplying cool gas in a pressure pattern that is non-uniform across the dimension of the glass sheets transverse to their direction of movement through a space between opposite arrays of modules disposed above and below the upper and lower major surfaces of the conveyed glass sheets. Non-uniform rates of cooling have developed non-uniform stress patterns, which are accompanied by optical non-uniformities, sometimes called Q-lines.
One technique for minimizing the appearance of Q-lines has been the application of blasts of air through narrow elongated slots, preferably narrower than one millimeter, extending continuously across the entire width of the conveyed glass sheets. Recognizing that it is difficult to maintain uniform width along the entire length of narrow slots, the prior art used thin mesh screens to separate the walls of the narrow slots and to maintain the uniformity of slot width. The presence of screens impaired the free flow of air through the slots and, hence, limited the heat transfer rate due to the impingement against the glass surface by gas streams flowing through the narrow slots en route to the glass surface. It became necessary to make the prior art modules hollow and to flow heat exchanging liquid through the hollow passages within the hollowed modules to improve the heat exchange rate by radiation. This solution introduced the problem of handling a liquid supply system.
Another technique for improving the uniformity of tempering medium applied across the entire width of glass sheets is the provision of longitudinally spaced modules having thin slots that extend obliquely of apertured, glass facing walls along the length of the modules so as to avoid the need for reinforcements interconnecting the walls of the modules that support the apertured walls, or for screens within the slots that help maintain a uniform width of slot. Such reinforcements break up the flow pattern of tempering medium en route to the major glass sheet surfaces and disrupt the uniformity of cooling pattern that would result if the flowing pattern were not interrupted locally. In addition, the oblique slots have limited length so that the module walls provide sufficient rigidity to maintain uniform slot width without requiring internal reinforcements or screens that interrupt free flow of tempering medium.
The use of thin slots permits a series of high velocity air jets which promote a high heat transfer coefficient at the major glass sheet surfaces using a given rate of flow of tempering medium. The oblique slots are arranged relative to the apertured walls of the modules in such a manner that adjacent slots of each module overlap one another along the length of the elongated bed. Thus, each glass sheet increment transverse to the path of glass sheet movement intercepts a plurality of oblique blasts imparted through oblique slots as it traverses the portion of said path in alignment with each of said modules. This arrangement provides substantially uniform cooling from transverse increment to transverse increment without requiring devices that impair the free flow of tempering medium as the cost of insuring uniformity of slit width over a long slot. The overlapping of slots along the path of glass movement causes the cooling of the glass surface to be sufficiently uniform as to minimize the development of Q-lines.
When glass must be tempered, a large escape area must be provided for the impinging blasts of cooling medium, such as air, to be released readily from the central portion of the gaseous bed to avoid the establishment of a non-uniform pressure profile across the width of glass sheets transverse to the direction of glass movement. Such pressure profile increases toward the center of the glass and causes the glass to develop one of two metastable conditions, one in which the center of the glass sheets bows upward and another in which the center of the glass sheets bows downward.
When glass is supported on a gaseous support, the thickness of the gas bed is maintained as thin as possible to enable the incoming gas streams to impinge on the glass surface as efficiently as possible rather than blending with the gas bed that is already present. Therefore, when the glass develops a bowed shape due to the metastable conditions described previously, there is insufficient room for the glass to be conveyed between the upper and lower arrays of modules that supply the gaseous cooling medium needed to cool the glass sufficiently rapidly to develop a stress pattern through the glass thickness that strengthens the glass sufficiently so that the glass develops at least a partial temper.
The final temper level in a glass sheet depends on the following variables:
1--Coefficient of thermal expansion of the glass in the viscosity range of 10.sup.10 to 10.sup.15 poises. PA0 2--Relaxation characteristics of the glass in the viscosity range of 10.sup.10 to 10.sup.15 poises. PA0 3--Heat conductivity and specific heat of the glass, including radiation characteristics. PA0 4--Glass thickness. PA0 5--Temperature distribution of the glass at instant cooling starts. PA0 6--Time spent in various cooling stages. PA0 7--Heat flux at the glass-tempering medium interface. Heat flux involves both heat transfer coefficient and the temperatures of the glass and the medium.
For a given glass composition, equal heat extraction at the surface and equal temperatures, thicker glass sheets attain a higher temper than thin glass sheets.
The higher the glass temperature at the onset of cooling (up to a certain value), the higher the final residual stress attained.
The lower the apparent heat conductivity (which includes radiation) the higher is the stress attained.
Temper levels are higher with higher heat extraction rates at the surface.
Higher residual stresses are obtained with glasses having larger coefficients of thermal expansion in the viscosity range of 10.sup.10 to 10.sup.15 poises.
Higher tempers result (up to a certain limit) by increasing the duration of exposure to rapid cooling.
Optical defects become more severe when glass sheets have greater variability in local heat flux rates.
The "faster" the relaxation characteristics, the higher is the temper level attained.
In prior art apparatus providing thin, elongated slots for the application of gas under pressure interspersed with elongated slots for gas removal, glass sheets tended to develop vibrations or flutter in their thickness direction as they were conveyed through the cooling area. The severity of these vibrations was greater for glass sheets of lesser thickness and for stronger streams of tempering medium. Thinner glass sheets have become more popular lately and stronger streams of tempering medium are generally needed to impart a desired level of temper in thinner glass sheets. Stronger streams are obtained by applying higher pressures of tempering medium to the modules for discharge through the elongated slots and/or by floating the glass sheets in closer spacing to the slotted walls of the modules as the sheets travel through the cooling area. From time to time, breakage rates increased during mass production operations. Modules providing individual, more diffuse patterns of tempering medium did not develop such strong vibrations and flutter. However, the presence of Q-lines in tempered sheets of less thickness made on apparatus having modules imparting individual patterns of tempering medium made it advisable to seek a solution to the Q-line problem other than one involving modules imparting more diffuse individual patterns.
Other attempts to avoid the presence of Q-lines involved the use of fluted modules that provide curved passages that guide streams of tempering medium against the opposite major surfaces of glass sheets travelling through the cooling area. The streams of tempering medium imparted through the openings of the fluted modules are relatively wide compared to the narrow slots of the modules with slot openings. Consequently, while tempering apparatus having fluted modules in the cooling area reduce the tendency for the tempered glass sheets to develop Q-lines, the degree of temper developed leaves something to be desired unless exceedingly large blowers are used to develop the pressure within the fluted module. In treating thinner glass sheets having a nominal thickness of 3 millimeters and less, these problems are increased.
Attempts have been made prior to the present invention to use fluted modules in the upstream portion of the cooling area of tempering apparatus and so-called "rosette" modules that impart discrete diffused patterns of tempering medium in the downstream portion of the cooling area. Such attempts have resulted in improved temper stresses in glass sheets cooled in such apparatus compared to those fabricated with apparatus having fluted modules disposed throughout the entire length of the cooling area. However, the incorporation of "rosette" modules resulted in the presence of Q-lines when the tempered glass sheets were inspected by exposure at a very oblique angle to a light path from a lamp.
2. Description of the Prior Art
U.S. Pat. No. 3,607,198 to Meunier et al discloses a method and apparatus for moving hot glass sheets and similar ribbons that are supported pneumatically out of contact with solid surfaces by establishing alternate zones of static and kinetic gas pressure along the path of sheet movement. Each zone extends substantially across the full width of the sheet. Cool air under pressure is applied through a first multiplicity of slots, which are parallel and extend continuously substantially the full width of the ribbon and a second multiplicity of exhaust slots for the exhaustion of air applied through the first multiplicity of slots. A pair of pressure applying slots is located between each consecutive two exhaust slots. The distance between the pressure applying slots of a pair is greater than the distance between each pressure applying slot and its adjacent exhaust slot. The total area occupied by the slots is not substantailly more than 10 percent of the whole glass sheet facing surface of the supporting bed. The pressure applying slots are preferably between 0.4 and 0.7 millimeter and need not be greater than 1 millimeter wide, and the exhaust slots are between 1.5 and 2 millimeters wide.
The Meunier et al patent is designed for annealing glass sheets. Hence, even if there is some backward flow of the support gas into the furnace, it is not so extensive as to impair the annealing process or to cause glass breakage as would be the case were the glass being tempered.
While this patent states its system can be used for tempering as well as annealing glass, it requires hollow passages in the slotted module housings and liquid to flow through the hollow passages to supplement the air cooling with radiation cooling. The small proportion of apertures in the Meunier et al apparatus makes it impractical for tempering by use of gas blasts exclusively. The need for a water supply system to supplement the gas supply system makes the Meunier et al apparatus awkward to use. The gas exhausted in the Meunier et al apparatus is recirculated. Such recirculation impairs the efficiency of the gas applied to cool the glass sheets unless the gas is cooled during its recirculation.
Furthermore, in handling sheets having a width on the order of 30 centimeters or more, it is difficult to maintain continuous uninterrupted slots of uniform width across the entire width of the glass sheets without reinforcing the modules containing slotted walls facing the opposite major glass sheet surfaces. The reinforcements, if in the form of wire mesh as in the Meunier et al apparatus, disrupt the continuity and uniformity of the flow of gas through the slots. If the reinforcements are solid members interconnecting the walls of the modules beneath the slotted walls, they have to be so close to the slots in the slotted walls to insure uniform width that they interrupt the continuity and uniformity of gas flow. Any substantial non-uniformity of gas flow imparts non-uniform cooling in an amount sufficient to cause Q-lines in the glass, particularly from modules installed in close proximity to the exit of the furnace.
Without the reinforcements or wire spacers in the slots of the module construction, the elongated slots develop non-uniform width which causes a non-uniform application of cooling medium even where the uniformity of flow of cooling medium is not interrupted by reinforcements. Therefore, the glass sheet heat treating art required a method of treatment different from and representing an improvement over that provided by the Meunier et al patent.
Belgium Pat. No. 787,880 to PPG Industries, Inc. discloses a method and apparatus for tempering glass sheets which contains spaced rows of so-called fluted modules, the glass facing walls of which are provided with a series of parallel arcuate vanes that cause streams of gaseous tempering medium to move in curvilinear paths that result in gas streams having a relatively large component of motion in the direction of glass sheet movement away from a furnace, and in a downstream direction of the path of glass movement where the gas streams impinge on the glass. The main purpose of directing the streams of cooling fluid downstream is to avoid flow of the cooling gas in an upstream direction into the exit portion of the furnace. Any upstream flow of cooling gas into the exit portion of the furnace cools the exit portion of the furnace and prevents the glass from developing sufficient heat for tempering and may also cause the glass sheets to leave the furnace exit at a non-uniform temperature. As a result, glass sheets insufficiently heated tend to break when subjected to streams of cooling gas downstream of the furnace exit.
Means is provided to adjust the effective exhaust area of the spaces between adjacent rows of modules. Different effective exhaust areas are most beneficial for different glass sheet thicknesses.
The tempering apparatus of the Belgium patent is composed of square modules, each provided with arcuate vanes that gradually change the direction of streams of cooling gas toward the major surfaces of the glass sheets from directions normal to the respective surfaces to directions oblique to the respective surfaces. Arcuate curving of the paths of movement for the cooling medium may cause some turbulence in the flow of cooling medium. Laminar flow is more efficient in chilling a glass surface than turbulent flow. The modules are arranged in rows 1 inch (25.4 millimeters) wide separated by spaces ranging from 1/4 inch (6.35 millimeters) to 3/4 inch (19.05 millimeters) depending on the thickness of glass sheets processed.
This patent is silent as to the width of the slots formed between adjacent vanes. However, the drawings would appear to indicate that each slot has a substantial width. Gas streams applied at a given rate of volume through slots of such width have less velocity than gas streams applied through thinner slots. Therefore, cooling by passing cold fluid through the wide arcuate slots between adjacent arcuate vanes requires very large blowing equipment that consumes much energy. Such high energy consumption provides an inefficient operation and is frowned upon.
French Pat. No. 2,024,397 discloses glass sheet tempering apparatus comprising one or a plurality of slotted nozzles providing oblique passageways for the passage of tempering medium either in a direction oblique to the plane defining the path of glass travel through a cooling area through slots extending along lines normal or oblique to the path of glass sheet travel to direct cooling medium toward the opposite glass surfaces. Passageways are provided for removing tempering medium in a direction parallel to the plane of the sheet and transverse to the path of glass travel after the medium impinges on the opposite major surfaces of the glass sheets as the latter pass through the cooling area. The passageways for removing tempering medium have restricted openings which inhibit the free removal of tempering medium from the vicinity of the respective glass surfaces. The presence of these restricted paths and the need to turn the blasts of tempering medium into directions normal to the component of blast movement parallel to the path of glass travel make the apparatus of the French patent less efficient than desired to remove the tempering medium from the glass after the tempering medium has cooled the glass surface. The glass sheets are conveyed either by roller discs that engage an edge of each glass sheet or rollers that engage a supported major surface of the glass sheets and also provide boundaries for exhaust passages for removing tempering medium. Various embodiments of nozzle arrangements are disclosed, such as modules with obliquely extending slots facing the upper glass sheet surface combined with individual modules that impart individual diffused patterns against the bottom glass sheet surface downstream of a pair of nozzles that face the opposite major surfaces of the glass sheets immediately downstream of the furnace exit.
U.S. Pat. No. 3,395,943 to Wilde discloses the use of gaseous streams for maintaining a gaseous support under a glass surface and directs additional gaseous streams against the exposed periphery capable of developing forces transversely of the sheet. Some of these additional gaseous streams are directed against the rear edge to propel the glass sheet forwardly and other gaseous streams are directed transversely to maintain the lateral position of the sheet.
U.S. Pat. No. 4,046,543 to George B. Shields discloses apparatus for cooling and tempering glass sheets that comprises spaced slotted modules that face the opposite surfaces of a glass sheet throughout the length of a cooling area. Cold tempering medium is applied through oblique slots that direct air blasts obliquely away from a furnace exit in a downstream direction of glass sheet movement. The slots are formed in walls of longitudinally spaced, transversely extending modules and are oriented to apply a downward component of motion to the air blasts that impinge against the opposite glass sheet surfaces to force the lower edges of the glass sheets against rotating discs that propel the glass sheets through the cooling area (or at least in the upstream portion of the cooling area).
Uniformly wide spaces provided in uniformly spaced relation between adjacent modules having uniformly spaced slot openings to insure escape of applied tempering medium are believed to provide a resultant effect that causes the glass sheets to flutter as they travel through the cooling area, according to the theory behind the present invention. Modifying the spacing and width uniformities introduces other problems that are hard to solve.
Despite all the patents enumerated, a need still existed to produce thin glass sheets having higher temper values combined with optical properties superior to those obtainable from the prior art and with fewer production losses. The present invention provides a novel combination of selected prior art features to attain such desired results.